By Pilar Suarez and Jordi RubioBARCELONA (Reuters) - A van plowed into crowds in the heart of Barcelona on Thursday and Spanish media reported at least 13 people were killed, in what police said they were treating as a terrorist attack.The death toll was reported by Cadena Ser radio, citing police sources. Police said some people were dead and injured but did not confirm the number of casualties. They said were searching for the driver of the van.Spanish newspaper El Periodico said two armed men were holed up in a bar in Barcelona's city center, and reported gunfire in the area, although it did not cite the source of the information.It was not immediately clear whether the incidents were connected.A source familiar with the initial U.S. government assessment said the incident appeared to be terrorism, and a White House spokeswoman said President Donald Trump was being kept abreast of the situation.Media reports said the van had zigzagged at speed down the famous Las Ramblas avenue, a magnet for tourists."I heard screams and a bit of a crash and then I just saw the crowd parting and this van going full pelt down the middle of the Ramblas and I immediately knew that it was a terrorist attack or something like that," eyewitness Tom Gueller told the BBC."It wasn't slowing down at all. It was just going straight through the middle of the crowds in the middle of the Ramblas."Mobile phone footage posted on Twitter showed several bodies strewn along the Ramblas, some motionless. Paramedics and bystanders bent over them, treating them and trying to comfort those still conscious.Around them, the boulevard was deserted, covered in rubbish and abandoned objects including hats, bags and a pram."We saw a white van collide with people. We saw people going flying because of the collision, we also saw three cyclists go flying," Ellen Vercamm, on holiday in Barcelona, told El Pais newspaper.El Pais said the driver of the vehicle had fled on foot.TOURIST DRAWEmergency services said people should not go to the area around Barcelona's Placa Catalunya, one of the city's main squares at the top of the Ramblas, and requested the closure of nearby train and metro stations.Prime Minister Mariano Rajoy said he was in contact with authorities, and the priority was to attend to the injured.The incident took place at the height of the tourist season in Barcelona, which is one of Europe's top travel destinations with at least 11 million visitors a year.Vehicles have been used to ram into crowds in a series of militant attacks across Europe since July 2016, killing well over 100 people in Nice, Berlin, London and Stockholm.Witness Ethan Spibey told Britain's Sky News: "All of sudden it was real chaos. People just started running screaming, there were loud bangs. People just started running into shops, there was a kind of mini-stampede where we were, down one of the alleyways."He said he had taken refuge with dozens of other people in a nearby church."They've locked the doors because I'm not sure whether the person who may have done it has actually been caught, so they've locked the doors and told people just to wait in here."Barcelona is the capital of the wealthy northeastern region of Catalonia, which plans to hold a popular vote on Oct. 1 on whether it should secede from Spain. It is in dispute with the central government, which says the vote cannot go ahead because it is unconstitutional.In recent weeks, threatening graffiti against tourists has appeared in Barcelona. In one video released under the slogan "tourism kills neighborhoods", several hooded individuals stopped a tourist bus in Barcelona, slashed the tyres and spray-painted the windscreen.The attack was the deadliest in Spain since March 2004, when Islamist militants placed bombs on commuter trains in Madrid, killing 191 people and wounding more than 1,800.(Reporting by Madrid newsroom, writing by Mark Trevelyan; Editing by Alison Williams and Nick Tattersall)